Electrical conductor for electron discharge tubes



1934- I. E. MOUROMTSEFF ET AL 1,982,317

ELECTRICAL CONDUCTOR FOR ELECTRON DISCHARGE TUBES Filed Sept. 27, 1928 INVENTORS fz'a ff/yoaromrfwyf a 7 ATTORNEY Patented Nov. 27, 1934 UNITED STATES PATENT OFFICE ELECTRICAL CONDUCTOR FOR ELECTRON DISCHARGE TUBES Pennsylvania Application September 27, 1928, Serial No. 308,659

4 Claims.

Our invention relates to conductors for electric currents and particularly to conductors adapted for carrying alternating high-frequency current.

One object of our invention is to provide inleading conductors adapted to carry current to the hot cathodes of electron-discharge tubes.

Another object of our invention is to provide inleading conductors for electron-discharge tubes adapted to efficiently carry simultaneously highfrequency and low-frequency currents.

A further object of our invention is to provide inleading conductors adapted to efllciently carry simultaneously high-frequency and low-frequency currents of large value to conductors sealed into the walls of electrical-discharge tubes, and to avoid transmitting mechanical stresses to the sealing means.

Other objects of our invention will be apparent upon reading the following specification, taken in connection with the drawing, in which Figure 1 is a longitudinal sectional view of an electron-discharge tube provided with currentinleading conductors in accordance with our invention, and

Figure 2 is a detail view in perspective and in section of one end of the tube shown in Fig. 1.

Fig. 3 is a cross-sectional view on lines III--III of Fig. 1.

Alternating currents of frequencies of 10 cycles per second or over are ordinarily generated in electron-discharge tubes having cathodes maintained at incandescence by current flowing from an auxiliary source of low frequency or direct current. In the case of such tubes designed to provide large power output, the current which must flow through the leads sealed into the tube walls becomes of very large value. These currents may be of two rather different sorts. The high-frequency current, comprising both the power-output and chargingcurrent components, becomes of very large magnitude in such tubes. The charging current, for example, increasing with both the voltage and the frequency at which the tube is operated. The cathode leads, moreover, are required to carry both the high-frequency current and also the low-frequency current from the auxiliary source employed to maintain the cathode at incandescence. Where large cathodes are employed, the latter current becomes of very considerable magnitude.

It has been customary, in the prior art, to construct inleading conductors by sealing a solid rod of some suitable metal, such as tungsten,

into the glass wall of the tube, and to braze or weld to the outer end of such wires, very close to the glass seal, a stranded or other flexible conductor of sufficient current capacity. The latter conductor is usually made by stranding relatively small wires both to provide such flexibility as well as to prevent the transmission to the fragile seals of mechanical stresses to which the outer ends of the conductors may be subjected, to facilitate connecting the tube leads to the binding posts of the transmitter stand and to minimize the effect of eddy currents which would be present were solid conductors of considerable diameter employed.

However, as is well known, high-frequency currents tend to flow in a relatively thin layer or skin about the outside of a conductor, and, where stranded conductors are employed, this makes it necessary for the current to either follow a spiral course having considerable self-induction or to flow from strand to strand, thus developing a substantial amount of heat. In consequence, stranded conductors offer to the flow of ultrahigh-frequency currents, a very considerable and undesirable impedance. On the other hand, lowfrequency currents or direct currents may be employed to heat the cathode; and this can flow perfectly well in a stranded conductor, since its skin effect is substantially negligible.

Difficulty is also experienced, in the case of stranded conductors, from the fact that the latter are readily broken, particularly at the point where they join the solid wire constituting the seal, and thus reduce the available current-carrying area of the leads. v

In accordance with our invention, we avoid the foregoing difficulties by providing a composite conductor consisting of a stranded core surrounded by a sheath of relatively thin metal of high conductivity. The resulting structure has sufficient flexibility to avoid the transmission of mechanical stresses to the seals, but the relatively fine stranded wires on the periphery of the underlying core are protected from breakage due to mechanical strain and abrasion. The conductive sheath forms a path of low impedance for the high-frequency components of the current, while the underlying core furnishes a highly conductive path for the relatively low-frequency heating current.

With the foregoing principles and purposes in mind, reference may be had to the drawing which shows a vacuum-tight bulb 1, enclosing a cathode tube 2 and a grid electrode 3. A portion 4 of the tube wall is made of thin metal, such as copper, and constitutes the anode. Tubes of the foregoing structure are well known in the art, and the method of construction requires no detailed explanation here.

In accordance with our invention, current is conducted to the hot cathode through a pair of conductors 8, which may be of tungsten or molybdenum, sealed through the glass wall at the bottom of a re-entrant stem '7. The cathode, which may be of tungsten, is welded to the interior ends of the wires 8. The wires '8 project a distance of about 1" beyond the glass of the seal and to their ends is welded or brazed a compos'iteconductor 6 embodying ourjinvention'. This conductor consists of a stranded cable whichmay be composed of relatively fine wires twisted together to form heavier strands, the latter being, in turn, twisted together, in rope fashion, to,form the cable. Closely surrounding the cable is a sheath of metal which may, like the underlying cable,

be of copper, and which is made long. enough to sheath the cable well beyond the end of the re-entrant stem into which the cathode leads are sealed. The sheath and cable are held together, adjacent to the leads 9, by a brazed, soldered or welded joint; and the outer end of the sheath is also brazed, soldered or welded to the cable or merely squeezed around the cable to hold it firmly in pla ce,if desired. The longitudinal seam in the sheath may likewise be welded, soldered or brazed, although this is, in many cases, not essential. It will usually be found dcsirable to provide a holder of some insulating material, such as impregnated fiber, to space the conducting leads apart at the mouthof the reentrant seal.

While we have described the sheath surrounding the underlying cable as being formed of sheet metal wrapped around the latter, it will be obvious that it may be applied thereto in other ways, as by electro-plating directly on the core in a suitable electrolytic bath. It may also be applied by means of .Schoop-spraying of metal on the core. While it will ordinarily be found desirable to form the sheath of the same metal as the underlying core, this is not absolutely essential and circumstances may arise when it will be found desirable to form the sheath and the core of different metals. I

It will be seen that, by the foregoing construction, a conductor is producedwhich is sufficiently flexible so that no mechanicalstrains, particularly side strains, can be transmitted through it to the fragile seals at the inner end of the reentrant stem. On the other hand. the surrounding sheath provides a path of high conductivity for the high-frequency currents which tend to flow therein, whereas the underlying cable provides a path of large area and high conductivity for low-frequency current.

While we have described our invention as applied to the cathode seals it will be obvious that, in many instances, it may be desirable to employ it for grid or other electrode seals, The principles of our invention are, in fact, of general applicability and many other specific structures for embodying them will be apparent to those skilled in the art. We, therefore, desire that the following claims shall be construed as broadly as is permissible in view of their express terms and of the limitations imposed by the prior art.

We claim as our invention:

1. In combination with a vacuum-tight envelope having a reentrant portion, a conductor sealed in said reentrant portion, a flexible connector joined to said first-named conductor and surrounded, for at least a portion of its length, by the reentrant portion, said flexible connector comprising a flexible conducting core surrounded by a flexible conductive sheath.

2. 'In combination with a vacuum-tight envelope, an electrode in said envelope, 'a conductor in the form of a solid rod sealed in the wall of said envelope and connected to said electrode, a flexible connector joined to the external terminal of said'first-named conductor, said flexible connector comprising a flexible conducting core surrounded by a flexible sheath of thin sheet metal.

3. In combination with a vacuum-tight enve" lope, an electrode in said envelope, a conductor inthe form of a solid rod sealed in the wall of said envelope and connected to said electrode,

a flexible connector joined to the external terminal of said first-named conductor, said flexible connector comprising a stranded conducting core surrounded by a flexible conductive sheath.-

4. In combination with a vacuum-tight envelope, an electrode in said envelope, a conductor in the form of a solid rod sealed in the wall of said envelope and connected. to said electrode, a flexible conductor connected to the external terminal of said first-named conductor, said second conductor comprising a. flexible conducting core surrounded by a flexible sheath of thin sheet metal of the same metal as said core.

ILIA E. MOUROMTSEFF. GREGORY RYLSKY,

its 

